The Colorado River, the lifeblood water supply of San Diego and the Southwest, made history late last year. And it wasn’t good.

Lake Mead, the reservoir holding the Colorado River back behind the Hoover Dam outside Las Vegas, hit its lowest point since being filled in the 1930s. Had it dropped just a few more feet, federal officials would’ve declared the first shortage in the river’s modern history. Arizona and Las Vegas would’ve gotten less water.

Fortunately, the lake’s decline stopped. Officials began a massive transfer of water from Lake Powell, the river’s other major reservoir, on the Utah-Arizona border. Along with a wet winter in the Colorado Rockies, that staved off what was once unthinkable. The Colorado River wasn’t supposed to come up short.

Though the Southwest got a reprieve, it was a troubling sign for a river relied on by 27 million people in places like San Diego, Los Angeles, Phoenix, Las Vegas and across the border in Mexico. The Colorado River today faces two serious threats, either of which would mean less water for the arid Southwest.

The first: The river’s annual supply was divided in the 1920s during a historically wet period. The Southwest created its expectations of the Colorado’s annual yield during one of the wettest centuries on record. If yields return to their historic average, there won’t be enough water to go around.

The second: Climate change is projected to cut the Colorado’s yield between 10 percent and 30 percent by 2050. Even a small reduction would have a major impact. In the lower Colorado basin, California, Arizona, Nevada and Mexico take nearly every last drop to which they’re entitled. Scientists at the Scripps Institution of Oceanography say a 10 percent drop could cause shortages in six of every 10 years.

Underlying those dire projections is a major uncertainty: If the Colorado consistently comes up short, no one knows who will cut consumption to keep Lake Mead from running dry. A short-term plan is in place from the time the reservoir hits 1,075 feet above sea level until 1,025 feet. Arizona and Las Vegas take that hit. (At 895 feet, the reservoir wouldn’t be able to distribute water and the Hoover Dam wouldn’t produce power. The lake is currently at 1,096 and climbing.)

Hitting those triggers would be monumental. At 1,025 feet, the reservoir would be a 25-foot drop away from cutting off Las Vegas. Someone would have to take less water to stop the decline. But there is no plan on how to ensure water gets to the big city in the middle of the desert — only an agreement that states will meet to negotiate. And with the vicious politics that can thrive around water, that meeting won’t be easy. It would require Mexico, California, Arizona or Nevada to give up some of their water.

“We do not believe that it is widely appreciated among the millions of people served by the Colorado River,” they wrote in a 2009 paper, “that the current emergency plan in the event of low water levels is, put baldly, to call a meeting.”

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It’s hard to overstate how important the Colorado River is to San Diego.

Since first tapping the river in 1947, San Diego has grown ever more reliant. After an early 1990s scare in which the Los Angeles-based Metropolitan Water District threatened to cut San Diego’s supplies by half, the region began working to diversify its water sources. Back then, San Diego was almost totally reliant on Metropolitan.

But as San Diego has turned away from that agency, it has turned more toward the Colorado. In 1991, about half of San Diego’s water came from the river. Last year, about 75 percent did.

Though San Diego has suffered through droughts and shortages twice in the last 20 years, its other major source has been responsible: the Sacramento Delta, the massive estuary that funnels Sierra Nevada snowmelt toward the Golden Gate Bridge. The Colorado stood strong.

But the Colorado has wavered over the last decade, and it’s impossible to pinpoint what impact a severe shortage on the river would mean for San Diego. If Lake Mead drops to 1,025 feet — federal officials say there’s a less than 5 percent chance it will in the next decade and a 10 percent chance in the next 20 years — no one knows what happens next. Not for San Diego or anyone else that relies on the river.

“It’s a good question. We’re not sure,” said Terry Fulp, deputy regional director for the federal Bureau of Reclamation, which manages the Colorado. “What we do know is that all of us have agreed to come back together, sit down and discuss additional measures we’d need to take to ensure the viability of Lake Mead and the resources it supports.”

Strategies for keeping Lake Mead from dropping further could involve paying farmers to fallow fields, requiring urban residents to conserve, negotiating an agreement with Mexico to reduce its share or striking a deal with states in the upper Colorado that don’t use all they’re entitled to today.

“It will not be easy, because there will be lots of resources at risk: hydropower, recreation, the environment, agriculture” and urban water users, Fulp said. “All of the resources will be at that table fighting for their piece. That will be a remarkable occurrence, too, not one I’m looking forward to.”

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The three states that rely on Lake Mead’s water aren’t tipping their hands.

If severe shortage strikes, it’s unclear whether California — and the agencies that supply San Diego — would take less water to keep Lake Mead from dropping ever closer to its bottom. Nevada and Arizona expect California to take a supply cut, for which California is not likely to volunteer, even though it gets the majority of the lower Colorado’s water and uses it for agriculture.

Something would have to give. Cutting off Nevada would mean leaving 2 million people there without the source of 90 percent of their water.

“We hope because we’re all in this together that it’s spread around, but nobody suffers a critical shortage as a result,” said J.C. Davis, a spokesman for the Southern Nevada Water Authority, which supplies Las Vegas. “It really is one watershed. It’s reasonable to expect everyone involved to be a part of the solution.”

Roger Patterson, assistant general manager of the Metropolitan Water District, a major supplier of San Diego’s water, said his agency doesn’t believe it should share in shortage — California was there first. Arizona didn’t finish building its aqueduct from the Colorado until 1993.

“Our view is that even at those levels we’re still senior to Nevada and Arizona on the river,” Patterson said. “What we want to do is continue to work together and do what we can to avoid getting there.”

Though they’re not legally required to negotiate until Lake Mead drops to 1,025 feet, water agencies say they’ll undoubtedly do so beforehand if the reservoir continues dropping. If Nevada and Arizona begin taking a supply cut, drastic changes will happen, said Halla Razak, Colorado River programs director at the San Diego County Water Authority, a major regional supplier.

“The states that agreed to take the shortage would simply not take it quietly,” Razak said. “Getting to that point is not a possibility almost. It’s not a likely scenario.”

San Diego has an advantage if shortage hits. The Imperial Irrigation District, which is supplying the region with a growing slice of Colorado water, has some of California’s most senior rights to the river’s water. That makes it less likely to be cut before other agencies, like Metropolitan, which has lower-priority rights.

As they look to a more arid future, water agencies say they’re trying to work cooperatively on Colorado issues, a break from the river’s contentious history. They’re jointly studying a desalination plant in Rosarito Beach with the Mexican government as a way to potentially lessen future shortages and entice Mexico to share any shortage.

Barnett, the Scripps scientist, delivers a dire warning about what will happen if that cooperation doesn’t continue. The worst-case scenario, he said, is that the states end up stuck arguing in court as Lake Mead runs dry.

“We are tied to it, and we’re stuck,” Barnett said. “You can conserve all you want, but push comes to shove and someone loses. Who’s that going to be?”

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Scott, Thanks for the comment and the thoughts. Taking your points one at a time: 1) The allocations for tribes are included in each state's figures. 2) Describing an acre-foot of water as enough to supply two homes for a year is generally regarded as imperfect but close to accurate. Obviously, water use varies from any one home to the next. It also varies from region to region. But 326,000 gallons, at least in San Diego, is enough for two average homes.

Scott, Thanks for the comment and the thoughts. Taking your points one at a time: 1) The allocations for tribes are included in each state's figures. 2) Describing an acre-foot of water as enough to supply two homes for a year is generally regarded as imperfect but close to accurate. Obviously, water use varies from any one home to the next. It also varies from region to region. But 326,000 gallons, at least in San Diego, is enough for two average homes.

It is either that, or we could just shut off the river below the dam and let Mexico go without, besides that might just shut down the drug trade and the cities on the Mexican side and solve our immigraton problems. The desert down there would become tem times as large which would make crossing it a death march. Problem solved. If they want water, they would have to police themselves to our satisfaction. Believe me, our southern problem would soon stop. WITHOUT WATER THERE IS NO CIVILIZATION !!!

It is either that, or we could just shut off the river below the dam and let Mexico go without, besides that might just shut down the drug trade and the cities on the Mexican side and solve our immigraton problems. The desert down there would become tem times as large which would make crossing it a death march. Problem solved. If they want water, they would have to police themselves to our satisfaction. Believe me, our southern problem would soon stop. WITHOUT WATER THERE IS NO CIVILIZATION !!!

2) You say 1MAF is "enough to supply 2 households for a year" - kind of ambiguous and as far as I've ever heard the statistic used to be 1 AF = 1 household family of 4 for 1 year (although I think it recently may have changed to 1 household of 4 for 1/2 year)

2) You say 1MAF is "enough to supply 2 households for a year" - kind of ambiguous and as far as I've ever heard the statistic used to be 1 AF = 1 household family of 4 for 1 year (although I think it recently may have changed to 1 household of 4 for 1/2 year)

I do however have a short term most important question. What if some or all of the dams along the Colorado go away? Is there any emergency preparedness for that? Despite the assertion of a climate change that pictures ever extending droughts and less and less availability of water, what is happening this year with the massive snowfalls in the mountains throughout the west (global warming = more water in the air) hints at a possible return to the 1983 nightmare that affected the Bureau of Reclamation which operates all the dams along the Colorado. It is no joke that they were concerned about keeping the Hoover Dam in that year. That scenario is being written again today as we speak. I would pay attention to it.

I do however have a short term most important question. What if some or all of the dams along the Colorado go away? Is there any emergency preparedness for that? Despite the assertion of a climate change that pictures ever extending droughts and less and less availability of water, what is happening this year with the massive snowfalls in the mountains throughout the west (global warming = more water in the air) hints at a possible return to the 1983 nightmare that affected the Bureau of Reclamation which operates all the dams along the Colorado. It is no joke that they were concerned about keeping the Hoover Dam in that year. That scenario is being written again today as we speak. I would pay attention to it.